Image orthicon target



March 24, 1959 G, B, `HARES 2,879,420

:MAGE oR'rr-IICON TARGET Filed Aug; `28. 1957 uan'nnunnau .4r/'offrir United States Patent IMAGE ORTHICON TARGET George B. Hares, Corning, N.Y., assignor'to Corning Glass Works, Corning, N.Y., a corporatlon of New York Application August 28, 1957, Serial No. 680,745

2 Claims. (Cl. 3134-68) This invention relates to glass compositions and particularly to compositions of glasses suitable for the target in lan image orthicon tu-be of improved design.

Such tubes are utilized in video cameras to convert an optical image into an electronic image for broadcasting purposes. Their general structure is well known to those skilled in the art pertaining to television and requires little amplication here. For a better understanding of the invention, however, reference is had to the accompanying drawing which is a schematic sectional view of an image orthicon tube showing its general structure.

In the drawing a glass tube or envelope generally designated has a plane front end or face plate 11 having on its inner surface a photo-cathode 12 (shown in exaggerated thickness) consisting of a thin, continuous, semitransparent layer usually comprising cesium, silver oxide and silver. Spaced from the photo-cathode is a glass target 13 (shown in exagerated thickness) and improved composition for which forms the subject of this invention.

In a typical image orthicon tube, the target 13 is a circular membrane, which may be about 40 mm. in diameter and 3-5 microns thick, sealed to a supporting ring 14 of metal having a thermal expansion coefficient compatible with that of the glass. Such glass, heretofore used for the target, was normally composed of a soda-limesilicate glass of the type used in making incandescent electric lamp bulbs, such as is described in Patent No. 1,369,988 and particularly in Example 4 of that patent.

In front of the target 13 and spaced about 50-100 microns therefrom is a very fine metal gauze 15 (shown in exaggerated thickness) having about meshes per mm. with about 50-65% open area. In the opposite end of the tube l0, an electron gun 16 is provided with a grid 17 surrounding the aperture ot' the gun and a multi-stage electron multiplier 1S.

In the operation of the image orthicon tube an optical image formed by a lens system (not shown) arranged in front of the face plate 11 is projected on the front surface of the photo-cathode 12. Photo-electrons thereby emitted from the rear surface of the photo-cathode strike the front face of the target 13 and cause the emission therefrom of secondary electrons in larger number than the photo-electrons. The emitted secondary electrons which are carried off by the metal gauze 15 leave electron deficiencies or positive charges on the front face of the target. Since the photo-electrons were compelled to pass through the metal gauze 15 on their way to the target 13, a pattern of individual positive charges thus remains on the front face of the target and delineates the image thereon.

Such charges are conducted perpendicularly but not laterally through the glass target to its rear face where they are neutralized by a scanning beam of low-velocity ielectrons provided by the electron gun 16. The electron beam thus modulated is reflected to the grid 17 causing; the emission of secondary electrons therefrom which are,

collected by the multiplier 18 and subsequently amplied for broadcasting.

Further details of the structure and operation of an image orthicon tube are described beginning at page of R.C.A. Review, volume lO (June 1949), in an article entitled Development and Performance of Television Camera Tubes, by Janes, Johnson and Moore.

The proper functioning of the glass target depends upon the rapid transfer of. the positive charges of the image perpendicularly through the thin glass membrane without substantial equalization or loss by transverse con- To prevent such transverse loss the glass the image by the scanning beam, a phenomenon known as sticking To facilitate the rapid removal ofthe image charge, the glass target is made as thin as possi! bie. Suitable targets have a volume resistivity equal to about 11.4 to 12.1 for log R at room temperature and a thickness of 3 to 5 microns.

ri`he composition of glass suitable as a target is dependent upon its interaction with the photo-cathode during operation of the tube. The glass target must not contaminate the photo-cathode and thereby decrease its ability to emit photo-electrons. Also the glass must be of such a composition that reaction with the photocathode does not cause undesirable effects such as sticking of the image. It has been found, that targets composed of the above-mentioned soda-lime-silicate glass tend to stick severely when used with improved photocathodes desirably containing a plurality of alkali metals. Likewise, it has been found that lithia-containing glass targets are unsuitable because they poison the improved photo-cathode although they do not form sticking images. Such improved photo-cathodes have higher sensitivities than prior photo-cathodes but have not been successfully incorporated into an image orthicon tube because glass targets suitable therefor have heretofore been unavailable.

lt is an object of the present invention to provide a glass composition suitable for a target in an image orthicon tube having a multi-alkali photo-cathode which will not form a sticking image nor poison the photo-cathode.

According to the invention such a glass consists essentially by weight of about 5%-65% SiO2, about 25% KZO, and about 10%-20% Tio-2.

While the stated proportion of KZO is critical within a fraction of a percent for maintaining the proper coefcient of thermal expansion, the proportions of SiO2 may vary from 55% to 65% and of TiOz from 10% to 20% without unfavorably affecting the thermal expansion coeiiicient and without unduly affecting the electrical resistivity and other physical properties.

Preferably, however, the glass consists essentially of about 60% SiOz, about 25% KZO and about 15% TiOz.

Examples of glass compositions which can be used for targets to obtain the ladvantages of this invention can be prepared by melting the following batches:

The glasses corresponding to the above batches, calcu- Patmehfed Man. .24, ,1.959.

lated in weight percent on the oxide basis, are as follows:

Sio 54. 7 59. 8 64. 7 X30 24. 9 24. 9 24. 9 T: 19. 9 14. 8 9. 9 Shao; 0. 0. 5 0. 5

The Sb203 functions only as a ning agent and has no substantial effect on the fundamental properties of the glass. The batches melt readily either in a closed pot'or in a tank, preferably under oxidizing conditions, at about 13001400 C. Preferably the glass should be stirred during melting and ning in order to minimize cord's. The' more important physical and electrical properties' ofthe above glasses are as follows:

What is claimed is:

1. In an image orthicon tube comprising a glass envelope containing in longitudinal arrangement a photocathode, a grid of fine metal gauze, a thin glass target and an electron gun, the improvement which comprises making said target of a glass consisting essentially by weight of %65% SiOz, about 25% KQO and 10%- 20% TOZ.

2. In an image orthicon tube comprising a glass envelope containing in longitudinal arrangement a photocathode, a grid of ne metal gauze, a thin glass target and an electron gun, the improvement which comprises making said target of a glass consisting essentially by weight of about SiO2, about 25% KZO and about 15% TiO2.

References Cited in the le of this patent UNITED STATES PATENTS 2,617,058 De Boer Nov. 4, 1952 2,617,954 Rose Nov. 11, 1952 2,676,109 Barnes Apr. 20, 1954 2,688,559 Armistead Sept. 7, 1954 UNITED STATES PATENT OFFICE CERTIFICATE OF lC0RRECTIO1\I Patent No. 2,879,420 March 24', 1959 George B Heres It is herebir certified that error appears in the-printed specification of the above 4numbered patent requiring correction and that the said Letters Patent should read as corrected below.

Column 2, line 4'?, for H5%=65%" read 55%-65% w; column 3, second table, first columr thereof, fourth line, after "Expansion coeff." for read X Signed and sealed this 9th day of' June 1959,.

litt) ROBERT C. WATSON Commissioner of Patents KARL H., MINE Attesting Oiicer 

1. IN AN IMAGE ORTHICON TUBE COMPRISING A GLASS ENVELOPE CONTAINING IN LONGITUDINAL ARRANGEMENT A PHOTOCATHODE, A GRID OF FINE METAL GAUZE, A THIN GLASS TARGET AND AN ELECTRON GUN, THE IMPROVEMENT WHICH COMPRISES MAKING SAID TARGET OF A GLASS CONSISTING ESSENTIALLY BY WEIGHT OF 55%-65% SIO2, ABOUT 25% K2O AND 10% 20% TIO2. 